//  CrowdingComparator.java
//
//  Author:
//       Antonio J. Nebro <antonio@lcc.uma.es>
//       Juan J. Durillo <durillo@lcc.uma.es>
//
//  Copyright (c) 2011 Antonio J. Nebro, Juan J. Durillo
//
//  This program is free software: you can redistribute it and/or modify
//  it under the terms of the GNU Lesser General Public License as published by
//  the Free Software Foundation, either version 3 of the License, or
//  (at your option) any later version.
//
//  This program is distributed in the hope that it will be useful,
//  but WITHOUT ANY WARRANTY; without even the implied warranty of
//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//  GNU Lesser General Public License for more details.
// 
//  You should have received a copy of the GNU Lesser General Public License
//  along with this program.  If not, see <http://www.gnu.org/licenses/>.
package jmetal.util.comparators;

import jmetal.core.Solution;

import java.util.Comparator;

/**
 * This class implements a <code>Comparator</code> (a method for comparing
 * <code>Solution</code> objects) based on the crowding distance, as in NSGA-II.
 */
public class CrowdingComparator implements Comparator {

    /**
     * stores a comparator for check the rank of solutions
     */
    private static final Comparator comparator = new RankComparator();

    /**
     * Compare two solutions.
     *
     * @param o1 Object representing the first <code>Solution</code>.
     * @param o2 Object representing the second <code>Solution</code>.
     * @return -1, or 0, or 1 if o1 is less than, equal, or greater than o2,
     * respectively.
     */
    public int compare(Object o1, Object o2) {
        if (o1 == null) {
            return 1;
        } else if (o2 == null) {
            return -1;
        }

        int flagComparatorRank = comparator.compare(o1, o2);
        if (flagComparatorRank != 0) {
            return flagComparatorRank;
        }

        /* His rank is equal, then distance crowding comparator */
        double distance1 = ((Solution) o1).getCrowdingDistance();
        double distance2 = ((Solution) o2).getCrowdingDistance();
        if (distance1 > distance2) {
            return -1;
        }

        if (distance1 < distance2) {
            return 1;
        }

        return 0;
    } // compare
} // CrowdingComparator
